A carton, box or container is typically made of a paper material, such as paperboard or the like, and includes a plurality of flaps at opposite open ends of the carton which must be folded and connected to provide a closed carton. Such cartons are usually provided in a flat, folded state to a manufacturer who then erects the carton, closes and seals one open end, inserts a desired product, and closes and seals the opposite end of the carton.
Machinery for erecting, folding and sealing a carton with a product therein are either intermittent or continuous motion systems. In an intermittent system, the carton is sequentially conveyed to a plurality of positions along an assembly line where the carton is stopped for a predetermined time period at each position so that a desired operation can be performed on the carton. In such a system, since the carton is substantially stationary for a short period of time, machinery can be provided at each position which is movable with respect to the stationary carton to provide the desired operation.
In a continuous motion system, cartons are continuously in motion, one after another, through the system. As the cartons move through the system, they cooperate with various apparatus which are substantially stationary with respect to the moving cartons and provide the desired operations on the cartons. Continuous motion systems dramatically increase the output approximately 5-10 times that of an intermittent system, and generally require completely different equipment.
Cartons are typically closed by folding end flaps of the carton using what is known as a "three surface conventional fold" method or pattern and then securing the flaps together with tape, an adhesive or with some other means. In a three surface conventional fold method, opposite minor end flaps are first folded, followed by a first inside major flap and finally a second outside major flap. Thus, as FIG. 3 illustrates, the carton ends are closed by three layers of flaps or surfaces, each of which extends across the open end and between which an adhesive layer is positioned.
When a carton contains a granular or powdered product, the powder can tend to work its way out of the carton between the folded and secured flaps during shipping and handling. One way to prevent such leakage is to provide the carton with an inner liner or pouch, such as a plastic bag. A liner, however, significantly increases the cost of the carton, can make the carton difficult to fill and seal with product, and can be difficult to recycle since most users neglect to remove and separate the carton and liner for respective recycling.
Attempts to prevent leakage without a liner include providing a "sift proof" carton which is preferably sealed with an adhesive. Sift proof adhesively sealed cartons rely on a precise predetermined pattern of a substantially quick setting adhesive and proper engagement between respective flaps.
Adhesively sealed sift proof cartons folded in accordance with a three surface conventional fold method have not been acceptable, particularly for relatively small grained powders, since such powders leak at the corners as well as at improperly sealed edges. Corner leakage primarily occurs from poor adhesion between the minor flaps and the first inside major flap, particularly at the proximal end of the first inside major flap which requires two corner seals of the carton. Edge leakage occurs due to the inability of existing methods and equipment to provide adhesive in exact locations with uniform consistency as explained in detail hereinafter.
Another way of closing a carton is in accordance with a two surface folding method which typically includes modifying the shape of the first inside major flap to enable direct contact between the minor flaps and the second outside major flap. Such methods tend to be more effective at preventing leakage of granular material since a sealing layer between the inside and outside major flaps is substantially eliminated as compared to the three surface conventional fold.
Existing two surface folding methods, however, which have been typically utilized in poly coated/flame bonded methods as well as intermittent systems, still exhibit poor corner sealing and edge sealing. In a poly coated/flame bonded method, the entire surface of each flap is coated with a polymer, such as plastic or the like, and then passed over a flame which melts the plastic and adheres the flaps together. In such methods, the plastic coating on all surfaces not only can be costly, but also is undesirable aesthetically and can have an adverse reaction with certain carton contents. The plastic coating can also create a limited line speed for some applications. Thus, the use of this method is limited.
As FIG. 15 illustrates, existing intermittent two surface folding methods utilize two distinct adhesive heads which move in predetermined patterns with respect to the stationary carton to provide two distinct adhesive patterns. One head applies a straight bead of adhesive on the inwardly folded minor flaps. The other head applies a substantially U-shaped bead of adhesive on portions of the minor flaps and extending about the periphery of the second outside major flap at a distance from the edge thereof. The U-shaped bead is provided by turning on the adhesive head at one end of the pattern and then depositing adhesive on the desired surfaces with at least two ninety degree turns. Thus, due to the motion required of the adhesive heads, such a method cannot be utilized on a continuous motion system.
As illustrated, the adhesive tends to glob when the head is turned on and off as well as at the corners and also tends to thin out proximate the center of each bead of adhesive. Such thinning occurs primarily due to the acceleration and deceleration of the adhesive head from the starting, stopping and turning motions.
Adhesive beads that are not uniform cause leakage because of the "open time" or curing of the adhesive selected for the particular application. In such an intermittent process, due to the time required to deposit the adhesive and advance the carton to the next station for folding engagement of the flaps, an adhesive with a substantially longer open time is preferably selected. Additionally, the adhesive must bridge the gap between the minor flaps and the outside major flap.
The open time is chosen so that upon contact of the adhesive with a particular folded flap, the adhesive is in a state where it grips the flap upon compression and sets so that upon release of compression the flap is secured. Accordingly, the dimensions of the bead as well as the selection of adhesive and timing of the system are important in obtaining a sift proof seal.
In view of the above, if the bead of applied adhesive is too thin, such as occurs toward the middle of the bead, the open time is decreased. Thus, the adhesive solidifies before making contact with the desired flap.
If the bead is too thick, such as where the globs occur, the larger mass of adhesive retains heat thereby lengthening the open time which fails to grip or hold a seal with the respective flap. In either event, a good seal is not obtained and leakage can readily occur.
It therefore would be desirable to provide a linerless adhesively sealed sift proof carton and method of assembly that can be utilized in an intermittent or continuous motion carton erecting and sealing system and can accommodate a variety of granular and powdered materials without leakage.